Advances in adenoviral vectors: from genetic engineering to their biology

Ad2 and Ad5 belong to a group of human cytolytic viruses that target the respiratory airways for reproduction, whereas latent infections establish within other tissues. Signals therefore exist that control this dichotomic process in different cell types, perhaps including cis and/or trans elements of viral origin. Since 1993, Ad2- and Ad5-based adenoviruses lacking all or part of the E1 regulatory region have been undergoing evaluation in phase I trials that target cancer and cystic fibrosis. These viruses are extremely attenuated and actually do not reproduce in most human cells. However, they retain most of the virus genetic program and often promote a significant cytotoxicity after infection, emphasizing the need to further cripple the virus biology to extend the duration of transgene expression, if required. We will review the strategies currently followed to engineer a professional lytic virus for epithelial cells into an innocuous gene delivery vehicle. Potential effects on the transducing properties of the vector that may result from the inactivation of viral activities that normally allow/regulate extrachromosomal gene expression during wild-type infection are discussed.